Japanese Unexamined Patent Application Publication No. 2009-040308 discloses a technology for, when estimating a road surface slope using a value detected by an acceleration sensor and a differentiated value of a vehicle speed determined by a wheel speed sensor, removing by using a low-pass filter noise components caused by changes in the attitude (squat, dive, shift shock) of a vehicle body or input disturbances of a road surface (such as irregularities).
In the technology of the related art described above, since the intensity of the filter is constant with respect to a change in road surface slope, the traceability of an estimated road surface slope to an actual change in road surface slope deteriorates as the rate of change of the road surface slope increases, resulting in a large estimation error.